1. Field of the Invention
This invention relates to an electro-acoustic transducer for converting electrical signals from, for example, a speaker, or collecting the sound from outside to convert it into electrical signals, in which the effect of the sound radiated to the back side of an electro-acoustic transducing device, such as a speaker unit, is eliminated to improve the acoustic characteristics from the mid to high ranges.
2. Description of the Related Art
As a speaker device, there has hitherto been known a bass-reflex type speaker device 50, shown in FIG. 23a, or an enclosure type speaker device 60, as shown in FIG. 23b. 
The bass-reflex type speaker device 50 has a duct 51 on the front side of a casing 53, as shown in FIG. 23a. The enclosure type speaker device 60, on the other hand, is of a hermetically sealed structure, without being formed with an opening, such as a duct, in a casing 63, as shown in FIG. 23b. 
The bass-reflex type speaker device 50 has sound pressure versus frequency characteristics A, impedance versus frequency characteristics B and second harmonics distortion versus frequency characteristics C, as shown for example in FIG. 24. It may be seen from the sound pressure versus frequency characteristics A in FIG. 24 that the sound pressure is decreased and increased in the low range and in the mid to high range, respectively. Correspondingly, the second harmonics distortion versus frequency characteristics C are increased in the low frequency range, while being lower in the mid to high range than in the low range.
The enclosure type speaker device 60 has sound pressure versus frequency characteristics A, impedance versus frequency characteristics B and second harmonics distortion versus frequency characteristics C, as shown for example in FIG. 25. It may be seen from the sound pressure versus frequency characteristics A in FIG. 25 that, as in the bass-reflex type, described above, the sound pressure is decreased and increased in the low range and in the mid to high range, respectively. Correspondingly, the second harmonics distortion versus frequency characteristics C are increased in the low frequency range, while being lower in the mid to high range than in the low range.
In the above speaker devices 50, 60, when the sound is radiated by diaphragms of speaker units 52, 62 towards the front side, the sound is radiated towards the back side of the speaker unit as well. In these speaker devices 50, 60, the radiated sound is reflected by the inner wall sections of the casings 53, 63 to return back to the diaphragms so as to be superimposed as noise components on the sound radiated from the diaphragms to deteriorate the acoustic characteristics.
In particular, in a speaker device having enclosure in the shape of a cube or parallelepiped, there are produced standing waves between inner wall sections facing the speaker unit. Moreover, significant noise components are superimposed on the sound radiated from the diaphragm.
It is an object of the present invention to provide a novel electro-acoustic transducer which is able to resolve the problems inherent in the conventional electro-acoustic transducer.
It is another object of the present invention to provide a novel electro-acoustic transducer having optimum acoustic characteristics free from adverse effects of the sound radiated from a diaphragm.
It is yet another object of the present invention to provide a novel electro-acoustic transducer having optimum acoustic characteristics free from adverse effects of the sound radiated from the back side of the diaphragm towards the inner side of the casing.
For accomplishing the above objects, the present invention provides an electro-acoustic transducer, such as a speaker device or an earphone device. The transducer includes an electro-acoustic transducing unit, such as a speaker unit, for converting input electrical signals into the sound, a casing in which the electro-acoustic transducing unit is arranged and which delimits a back cavity towards the rear side of the electro-acoustic transducing unit, and at least two openings having a sound duct communicating with the back cavity. The sound radiated from the back surface of the electro-acoustic transducing unit is transmitted into the inside of the sound duct thereof to prevent the sound from being again admitted into the inside of the electro-acoustic transducer.
The present invention also provides an electro-acoustic transducer including electro-acoustic transducing means for converting input electrical signals into sound, a casing in which the electro-acoustic transducing means is arranged and which delimits a back cavity towards the rear side of the electro-acoustic transducing means, and a plurality of sound ducts, with the sound ducts having respective one ends communicating with the back cavity and also having respective opposite ends communicating with one another. The sound radiated from the back surface of the electro-acoustic transducing means is transmitted into the inside of the sound duct thereof to prevent the sound radiated from the back side of the electro-acoustic transducing means from being again admitted into the electro-acoustic transducer.
The present invention also provides an acoustic-electrical transducer including acoustic-electrical transducing means for converting an input sound into electrical signals, and a sound duct having at least two openings and adapted for communicating with the rear side of the acoustic-electrical transducing means. The sound duct transmits to the sound radiated from the rear side of the acoustic-electrical transducing means into the sound duct for attenuation to prevent the sound radiated from the back side of the electro-acoustic transducing means from being again admitted into the electro-acoustic transducer.
Other objects and advantages of the present invention will become apparent from the following description of the present embodiments of the invention in conjunction with the drawings.